1. Field of the Invention
This invention relates to nuclear power and particularly to the control of recirculating steam generators in pressurized water nuclear steam supply systems (NSSS). More specifically, the present invention is directed to automatic water level controls for steam generators of nuclear power systems. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The nuclear steam generator of a pressurized water nuclear power plant is typically controlled as a function of three primary operating parameters which are monitored, i.e., water level (L) steam flow (W.sub.S) and feedwater flow W.sub.fw). The signals corresponding to the monitored parameters are processed in proportional/integral and lead/lag circuits to generate a feedwater flow demand signal for controlling the amount of water introduced into the steam generator for the production of steam. The principal concern, and therefore the operating parameter on which the control action is primarily based, is the steam generator water level.
In practice, the control of the steam generators of NSSS has proven to be an unusally difficult task. As a result, a significant proportion of major nuclear power plant outages have been caused by reactor trips due to steam generator operation outside the desired range. Many of these outages are due to reactor trips on low or high steam generator water levels. Typically, about 80 percent of steam generator low water level trips occur below 20 percent system rated power, and nearly 90 percent of the high water level trips occur below 20 percent power. The problem of maintaining steam generator water level within proper limits is particularly acute during plant startup, when the operators have had relatively little experience in steam generator water level control.
A major complexity incident to steam generator control, particularly at low power levels, resides in the water recirculation characteristics of the system including the steam generator. Thus, during low power operation, the sensitivity of the steam generator water level to changes in feedwater flow increases. Also, at low power there is a seemingly anomolous behavioral characteristic which is manifested by an initial decrease in steam generator water level when there is an increase in the feedwater flow. This behaviour often confuses the operator, and usually causes the operator to further increase the feedwater flow, causing a further decrease in the water level and introducing "positive feedback" into the system which may lead to uncontrolled oscillation of the water level and to a reactor trip.
Conventional controllers, even of the above-mentioned three parameter type, are unreliable at low power operation because the steam flow and feedwater flow signals are themselves not reliable under such operating conditions. In most instances, because of this known lack of reliability, the operators elect to manually control the water level. Attempts at manual control have met with only limited success to date.